1. Field of the Invention
The present invention relates to a semiconductor element.
2. Background Art
Heterojunction type bipolar transistors (hereinafter, referred to as “HBT”) used for conventional high frequency power amplifiers are generally configured as shown in FIG. 5. FIG. 5 is a diagram of a semiconductor element having a typical HBT structure illustrating problems to be solved by the present invention. The HBT shown in FIG. 5 is provided with an emitter layer 111 formed of an N-type semiconductor, an emitter electrode 112, a base electrode 113, a base layer 114 formed of a high concentration P-type semiconductor, a collector layer 115 formed of an N-type semiconductor, a collector ohmic layer 116 formed of a high concentration N-type semiconductor and a collector electrode 117.
In this HBT, a base-collector junction 118 is formed between the P-type base layer 114 and the N-type collector layer 115. In order to reduce a junction capacitance (Cbc) and improve high frequency characteristics (maximum oscillating frequency fmax or the like), this base-collector junction 118 is preferably as small as possible. For this reason, it is preferable to minimize the size of a PN junction end 119 located outside the base electrode 113.
The base layer may be etched by wet etching or dry etching When wet etching is used, dihydroxysuccinic acid/hydrogen peroxide or phosphoric acid/hydrogen peroxide may be used. When this base layer is worked out, care needs to be exercised to ensure that the base electrode may be avoided from the etched region. For this reason, an etching edge needs to be formed at a position away from the base electrode to a certain extent, and more specifically, a design needs to be conducted for providing an interval of 0.5 μm or more in consideration of dimensional controllability over a resist used during etching and a problem with accuracy of alignment with the base electrode. Thus, it is practically impossible to provide an etching end of the PN junction end 119 directly below the base electrode 113.
Note that when using dry etching, a technique may also be used whereby the base layer is worked out through dry etching with self-alignment using the base electrode as a mask so as to obtain a minimum base-collector junction area. However, Au is generally used as a base electrode material, and etching is performed using Au as a mask in this case. This may result in a problem with Au contamination of the dry etching apparatus or a problem that the device itself is contaminated by Au during etching, and therefore etching is not a process that allows mass production, and the aforementioned wet etching is in the mainstream in mass production sites.
Furthermore, in the HBT structure shown in FIG. 5, like an end 1110, the base-collector junction is exposed on the surface at the end where the base layer is worked out. For this reason, there are problems that the surface state is not stable and a leakage current between the base and collector is not stable or the like.
In this regard, as disclosed, for example, in Japanese Patent Laid-Open No. S60-164358, there is a technique whereby ions are injected into a base-collector semiconductor layer formed outside a base electrode using the base electrode as a mask to thereby produce inactivation.
However, the above-described conventional technique achieves inactivation through implementation of high resistance by injecting hydrogen ions (H+), oxygen ions (O+), boron ions (B+) into the base-collector semiconductor layer formed outside the base electrode. For example, H+ remaining in the base layer has an influence on impurities in the base layer, causing a current gain β of the HBT to fluctuate. O+ and B+ may become impurities, resulting in a problem with insulating properties.
Thus, the above-described conventional inactivation technique is a technique yet to be improved from the perspective of securing reliability of a semiconductor element by stabilizing a side face of the semiconductor layer.